As the use of computers continues to increase at a rapid rate, the demand for peripherals and systems connected via wireless connections continues to increase. The number of wireless applications is currently increasing at a very high rate in areas such as security alarms, networking, data communications, telephony and computer security.
Wireless communications currently may take many forms such as ultrasonic, IR and RF. A commonly used communication technique in RF wireless communications is spread spectrum. Spread spectrum communication is a communication technique whereby the transmitted signal is spread over a frequency band that is significantly wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference and jamming thus enabling high levels of data integrity and security. Further, since the signal spreading process spreads the transmission power over a wide bandwidth, the power levels at any given frequency within the bandwidth are reduced significantly thereby reducing interference to other radio devices.
Spread spectrum communication systems are generally of the direct sequence (DS) type, the frequency hopping (FH) type or are a hybrid of the two that combines DS and FH. In direct sequence spread spectrum communications, a data signal is modulated with a pseudo random chip code so as to generate a transmitted signal whose frequency spectrum is spread over a wide bandwidth. The transmitted signal has a low spectral density and appears as noise to receivers lacking the code sequence. Thus, spread spectrum communications provides increased security for the data transmitted and reduced interference with other transmitters and receivers operating in the same environment.
The role of the transmitter in a spread spectrum communications system is to spread the signal in accordance with the data to be transmitted. Each bit or set of bits to be transmitted is converted into a plurality of chips having a much wider bandwidth than the original data. The spreading is performed in accordance with the code sequence chosen for the system.
The role of the receiver is to de-spread the spread spectrum signal in order to recover the original data signal. In direct sequence spread spectrum, the de-spreading of the signal is accomplished by correlating the received signal with a reference code matching the pseudo noise code used by the transmitter to transmit the information. As a consequence of de-spreading the signal, any interfering signals are also spread. The interfering signals typically comprise pseudo-random noise rather than cyclic noise that is easier to combat.
One technique for spread spectrum correlation is to convert the received signal into digital form before inputting it to a digital matched filter. Other spread spectrum correlation techniques utilize surface acoustic wave (SAW) devices to perform correlation on a received spread spectrum signal. SAW devices, constructed on quartz wafers having a thickness of 0.5 mm, permit propagation of acoustical waves on the free surface. The SAW device functions to convert electrical signals into acoustical signals and back again via piezo electric transducers.
SAW devices are useful in a variety of applications including spread spectrum correlators since they are generally capable of operating over a wide bandwidth. A SAW correlator device is a passive component constructed to recognize a specific sequence of code chips (similar in operation to a digital matched filter correlator) via correlation of phase shifts in an RF signal. The SAW correlator functions analogously to a delay line matched filter. It consists of many delay elements each having a delay period equal to the period of the transmitted code clock such that, at any time, each element corresponds to a single chip of the received signal.
As the received signal propagates through the SAW device, the phase structure of each element is added in or out of phase with the propagated wave. The outputs of all the elements may be summed to reach a maximum at a total correlation value. When the phase shift structure of all the elements matches the phase shifts of the propagated wave, a maximum sum, i.e., correlation, is achieved.
Since SAW devices are by nature fixed devices, a SAW correlator is usually programmed at the time of manufacture to match a single predetermined chip code sequence. The phase shift structure of the SAW device is programmed at the time of construction Through transducers placed in each element to produce an elemental phase match and cannot be changed once manufactured. Thus SAW devices generally permit correlation with a single code sequence.
Prior art SAW based transceivers that use SAW technology for the spreading and de-spreading function are mostly based on a single central frequency that is fixed. An advantage of such a fixed single frequency system is that it is relatively simple, straight forward, easy to implement and permits a fairly quick wake up time. A major drawback to such systems, however, is that the transmission frequency is fixed. The transceiver was thus limited to operation at only one frequency yielding a single operating channel. Operating with only a single channel poses several challenges in areas that are crowded with RF transmissions. In the event other transmissions occupy the single operating channel, interference is inevitable with the potential for a blocked channel. In extreme cases, the inability to hop to an alternative frequency channel can potentially disable the wireless communications link altogether.
It would therefore be desirable to have an RF modem that utilizes direct sequence spread spectrum techniques that has frequency agility, is simple to implement and can be constructed at low cost and small size. It is also desirable that such an RF modem utilize a SAW device for the transmitter correlator and receiver correlator thereby reducing the size and cost of the modem.